The first principal aim of the proposed work is to develop so-called switching ligands, thereby providing an unusual strategy for working on som of the most important reactivity/selectivity problems in modern chemistry. Switching ligands are ligands which coordinate to a primary atom-transfer site, an oxo-site in the proposed examples, and have a secondary site with a separate function. This function is the reversible acquisition of charge such that the primary site can be chemically changed by a protonation/deprotonation or an electron-out/election-in event at the secondary site. Switching ligands offer an opportunity to develop metal systems with chameleon-like electronic properties, being capable of reactin with mild oxo-forming reagents to produce, after a switching event, a reactive O-atom transfer agent. The proposed switching process might allow both the kinetic and thermodynamic features of an atom-transfer event to be significantly altered. One way of viewing the challenge of achieving functioning metallooxidants based on switching ligands is to see it as one not of mimicking biological catalysts, but rather one of moving towards the complexity of function of atom-transfer enzymes employing multifunctional small molecules. The second principal aim is to exploit a series of strongly donating, oxidation resistant ligands developed to stabilize high valent metal centers. These ligands offer the opportunity to synthesize an isolate as crystalline materials a range of stable nonheme iron(IV) species and study the molecular and electronic structures in great detail. The target complexes contain a range of ligands of central significance to the structure and function of iron enzymes. A library of structures and spectroscopic parameters (Mossbauer, ESR, and spin Hamiltonian parameters, vibrational data, EXAFS, NMR, magnetic data) and electrochemical data will be collected to serve as reference material for the reactive centers of iro metalloproteins. The two principal aims converge in a body of proposed atom-transfer reactivity.